When parties to a contract make promises to perform their obligations, and one party reasonably relies on the other party’s promise—but the party making the promise fails to perform, causing harm or loss to the party who relied on the promise—the party who relied on the promise to perform is said to have relied to its detriment.

This legal concept is called detrimental reliance. Detrimental reliance may serve as a substitute for consideration, and make an otherwise unenforceable contract enforceable.

Thus, detrimental reliance is a legal concept based on fairness (known as equity or equitable), and is equivalent to contractual promissory estoppel (due to the other party’s reliance, the party who did not keep its promise is prohibited from challenging the enforceability of its promise).

Detrimental reliance is not a separate tort cause of action.

In Pennsylvania, the legal concept of detrimental reliance is recognized and is often referred to as promissory estoppel. Under this doctrine, if one party makes a promise that they reasonably expect to induce action or forbearance on the part of the promisee, and the promisee does indeed take such action or forbearance to their detriment, the promisor may be estopped from reneging on the promise, even if the original agreement lacked consideration and would not constitute an enforceable contract under traditional contract principles. This concept is rooted in principles of fairness and aims to prevent injustice that would result from the promisor's failure to fulfill their promise upon which the other party has relied. Detrimental reliance serves as a means to enforce certain promises that have caused one party to change their position in reliance on the promise made by the other party. It is important to note that while detrimental reliance can make an otherwise unenforceable promise enforceable, it is not considered a separate cause of action in tort law in Pennsylvania.